1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing a supercharger rotor.
2. Description of the Related Art
FIG. 1 is a schematic view of a supercharger rotor. The supercharger rotor comprises male rotor (M rotor 1) and female rotor (F rotor 2) rotated while being engaged with each other. The male rotor 1 includes a plurality (three in the drawing) of helical convex portions 1a, and the female rotor 2 includes helical concave portions 2a engaged with the helical convex portions 1a with no gap. Gas (e.g., air) is compressed between the helical convex and concave portions 1a and 2a, and the air is pressurized to supercharge in an internal combustion engine.
The supercharger rotor also comprises a profile portion 3 having the helical portions 1a and 2a, and a shaft 4 penetrating the profile portion 3. The profile portion 3 is normally made of aluminum, and the shaft 4 of steel. Accordingly, in order to firmly connect the profile portion 3 with the shaft 4, conventionally, metal bonding means have been employed to execute aluminizing for the shaft side, and connecting the shaft made of steel with the profile portion made of aluminum. In this case, since the shaft 4 and the profile portion 3 are connected with each other by metal bonding, the rotor must be maintained at a high temperature for a long time.
Conventionally, the supercharger has been manufactured by gravity casting or precision casting.
The gravity casting is a method of manufacturing a rotor by pouring molten metal (hot metal) into a mold, and solidifying it. For the mold, a sand mold or a metal mold is most often used. The mold has a cavity portion equivalent to a product (rotor in this case), and hot metal can be poured into this portion.
For the gravity casting, in the case of mass production, automization has been pursued in various manners. Still, however, manufacturing of a die or its disassembling takes time (e.g., about 6 min.), lowering productivity. Since feeder head twice as much as a product is necessary, lowering yield, and increasing costs. Because of low accuracy of a casting, an excess thickness of about 3 mm is necessary, accordingly increasing a processing margin, which result in longer processing time, and higher processing costs. Further, it is difficult to provide a helical hollow portion inside the rotor having the helical portion, consequently making the rotor heavy. Thus, the conventional rotor has many drawbacks such as a large moment of inertia, unsuitable for high-speed rotation and operation stop characteristics, and low response to an engine speed.
On the other hand, the precision casting is a shell mold method or a lost wax method, and characterized by high accuracy of a casting. However, it is substantially impossible to manufacture a rotor by the shell mold method. In addition the lost wax method includes many steps, lowering productivity, and increasing costs. Further, although the helical portion can be made hollow or the shaft can be cast-coated, costs are higher.
In order to solve the above-described connection problem by the aluminizing, means has been provided to fix a profile portion and a shaft to each other by a pin, or provide a groove 5 in a shaft 4, and cast-coat it as shown in FIG. 2A (Japanese Patent Application Laid-Open No. 301211/1995), or means has been presented to provide a through-hole 6 in a shaft 4, and cast-coat it (Japanese Patent Application No. 49677/1996). In these means, however, problems of high costs caused by increases in processing steps and components have been inherent.